linux-kernel - RE: [PATCH 1/1] clocksource: hyper-v: Rework clocksource and sched clock setup

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Date:   Mon, 19 Jun 2023 16:44:47 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH 1/1] clocksource: hyper-v: Rework clocksource and sched
 clock setup

From: Daniel Lezcano <daniel.lezcano@...aro.org> Sent: Monday, June 19, 2023 9:16 AM
> 
> On 09/06/2023 17:47, Michael Kelley wrote:
> > Current code assigns either the Hyper-V TSC page or MSR-based ref counter
> > as the sched clock. This may be sub-optimal in two cases. First, if there
> > is hardware support to ensure consistent TSC frequency across live
> > migrations and Hyper-V is using that support, the raw TSC is a faster
> > source of time than the Hyper-V TSC page.  Second, the MSR-based ref
> > counter is relatively slow because reads require a trap to the hypervisor.
> > As such, it should never be used as the sched clock. The native sched
> > clock based on the raw TSC or jiffies is much better.
> >
> > Rework the sched clock setup so it is set to the TSC page only if
> > Hyper-V indicates that the TSC may have inconsistent frequency across
> > live migrations. Also, remove the code that sets the sched clock to
> > the MSR-based ref counter. In the cases where it is not set, the sched
> > clock will then be the native sched clock.
> >
> > As part of the rework, always enable both the TSC page clocksource and
> > the MSR-based ref counter clocksource. Set the ratings so the TSC page
> > clocksource is preferred. While the MSR-based ref counter clocksource
> > is unlikely to ever be the default, having it available for manual
> > selection is convenient for development purposes.
> >
> > Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> 
> The patch does not apply, does it depend on another patch?

It should apply to linux-next.  It depends on two previous patches from
Peter Zijlstra in the sched/core branch of tip.  See:

https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=sched/core&id=9397fa2ea3e7634f61da1ab76b9eb88ba04dfdfc

and 

https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=sched/core&id=e39acc37db34f6688e2c16e958fb1d662c422c81

Michael

> 
> Rejected chunk:
> 
> --- drivers/clocksource/hyperv_timer.c
> +++ drivers/clocksource/hyperv_timer.c
> @@ -485,15 +485,9 @@ static u64 notrace read_hv_clock_msr_cs(struct
> clocksource *arg)
>   	return read_hv_clock_msr();
>   }
> 
> -static u64 noinstr read_hv_sched_clock_msr(void)
> -{
> -	return (read_hv_clock_msr() - hv_sched_clock_offset) *
> -		(NSEC_PER_SEC / HV_CLOCK_HZ);
> -}
> -
>   static struct clocksource hyperv_cs_msr = {
>   	.name	= "hyperv_clocksource_msr",
> -	.rating	= 500,
> +	.rating	= 495,
>   	.read	= read_hv_clock_msr_cs,
>   	.mask	= CLOCKSOURCE_MASK(64),
>   	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
> 
> 
> --